Android Pie has been out for about two weeks now. And developers have been hard at work in bringing the latest version of Android over to older smartphones that are no longer being supported, like the Motorola and LG-made Nexus 6 and 5X. The last official OS upgrade that the Nexus 6 received was Android Nougat, and the Nexus 5X got Android Oreo. But neither are getting Android Pie, officially from Google. But some developers have created custom ROMs bringing Android Pie to these two devices.
These custom ROMs are pretty new, and will definitely have some bugs in them. So if you are fine with a few bugs here and there, then it's definitely worth checking out. It's also worth noting that where Qualcomm isn't releasing binaries for these processors any longer, Android Pie is going to be pretty janky on these smartphones. After all, the Nexus 6 launched four years ago, and the Nexus 5X was three years ago. Not to mention, these processors and other internals do age, and they don't always age gracefully. So keep that in mind before you do decide to flash these custom ROMs. Also remember that flashing a custom ROM will void your warranty, but given the age of both smartphones, you are likely out of your warranty already anyways. So that shouldn't be an issue.
If you are interested in flashing either of these custom ROMs, make sure you hit up the links below. Both ROMs have respective threads on XDA Developers Forum, which detail everything about these ROMs which includes any bugs or known issues on these devices. It's still rather impressive to see these devices getting stable ROMs in 2018. A few years ago, you would see custom ROMs for older devices that were missing Bluetooth support, or the charging port wouldn't work. But these are relatively stable in that regard.